fix HomeView N+1 and cache the curriculum tree

GET /api/home is the most-hit authenticated endpoint (every app open) and
was running one COUNT per node. Now it is a flat number of queries
regardless of grade size, and the shared curriculum skeleton is served
from cache; per-user progress/stats stay live.

content/views.py
  - _node_status(): takes a precomputed set of completed lesson_ids and a
    node's lesson_ids instead of firing its own COUNT query.
  - HomeView.get(): one query for all completed lesson_ids of the user in
    the grade, status computed in Python; grade tree (units -> nodes ->
    lesson_ids, no per-user data) read from cache
    (content:tree:grade:{id}:v{N}); missing grade -> 404 (was an
    unhandled 500); grade with no units -> empty units, not 404.
  - GradesListView: served from cache (content:grades:v{N}).
  - curriculum_version() / bump_curriculum_version(): one shared version
    key; bumping it invalidates every content cache key at once.
  - both GET views get throttle_scope='read' + GuestAwareContentThrottle.
content/receivers.py
  - post_save/post_delete on Grade/Unit/Node/Lesson -> bump_curriculum_version().
content/tests.py
  - HomeViewScalabilityTests: query count flat for 2 vs 20 nodes; 2nd call
    hits cache (no content_unit/node SQL); a new Node invalidates the tree;
    missing grade 404; empty grade 200.
  - GradesListCacheTests: list cached, invalidated on new Grade.
  - ReadThrottleTests: /api/home returns 429 past the limit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

content/tests.py

@@ -541,3 +541,114 @@ class QuestionAnswerMascotMoodTests(TestCase):
         )
         self.assertEqual(first.data['mascotMood'], 'encourage')
         self.assertEqual(replay.data['mascotMood'], 'encourage')
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# مقیاس‌پذیریِ HomeView: رفعِ N+1، کشِ برنامه‌ی درسی، ۴۰۴ گریدِ ناموجود
+# ─────────────────────────────────────────────────────────────────────────────
+from django.core.cache import cache as _django_cache
+from django.test.utils import CaptureQueriesContext
+from django.db import connection
+
+
+class HomeViewScalabilityTests(TestCase):
+    def setUp(self):
+        _django_cache.clear()
+        self.client = APIClient()
+        self.user = User.objects.create(device_id='dev-home-scale', is_guest=True, name='')
+        self.token = issue_access_token(self.user.id)
+
+    def _grade_with_nodes(self, n_nodes):
+        grade = Grade.objects.create(title=f'g{n_nodes}', icon='i', color='#fff')
+        unit = Unit.objects.create(grade=grade, title='u', order=1)
+        for i in range(n_nodes):
+            node = Node.objects.create(unit=unit, title=f'n{i}', order=i)
+            Lesson.objects.create(node=node, title=f'l{i}', intro_text='.')
+        return grade
+
+    def _home(self, grade_id):
+        return self.client.get(
+            f'/api/home?grade={grade_id}', HTTP_AUTHORIZATION=f'Bearer {self.token}')
+
+    def test_query_count_is_flat_regardless_of_node_count(self):
+        g_small = self._grade_with_nodes(2)
+        g_big = self._grade_with_nodes(20)
+
+        _django_cache.clear()
+        with CaptureQueriesContext(connection) as q_small:
+            self.assertEqual(self._home(g_small.id).status_code, 200)
+        _django_cache.clear()
+        with CaptureQueriesContext(connection) as q_big:
+            self.assertEqual(self._home(g_big.id).status_code, 200)
+
+        # قبلِ رفعِ N+1، q_big چند برابرِ q_small بود (یک COUNT به‌ازای هر node).
+        self.assertEqual(len(q_small), len(q_big))
+
+    def test_second_call_hits_cache_not_curriculum_tables(self):
+        g = self._grade_with_nodes(5)
+        self.assertEqual(self._home(g.id).status_code, 200)  # پر کردنِ کش
+
+        with CaptureQueriesContext(connection) as q:
+            self.assertEqual(self._home(g.id).status_code, 200)
+        sql = ' '.join(e['sql'] for e in q).lower()
+        self.assertNotIn('content_unit', sql)
+        self.assertNotIn('content_node', sql)
+        # فقط کوئریِ per-user باید بمونه
+        self.assertIn('userlessoncompletion', sql.replace('_', ''))
+
+    def test_content_change_invalidates_tree_cache(self):
+        g = self._grade_with_nodes(3)
+        self._home(g.id)
+        unit = g.units.first()
+        Node.objects.create(unit=unit, title='new-node', order=99)  # post_save → bump version
+
+        r = self._home(g.id)
+        node_titles = [n['title'] for u in r.data['units'] for n in u['nodes']]
+        self.assertIn('new-node', node_titles)
+
+    def test_missing_grade_returns_404(self):
+        r = self._home(999999)
+        self.assertEqual(r.status_code, 404)
+
+    def test_grade_with_no_units_returns_empty_not_404(self):
+        g = Grade.objects.create(title='empty', icon='i', color='#fff')
+        r = self._home(g.id)
+        self.assertEqual(r.status_code, 200)
+        self.assertEqual(r.data['units'], [])
+
+
+class GradesListCacheTests(TestCase):
+    def setUp(self):
+        _django_cache.clear()
+        self.client = APIClient()
+
+    def test_list_is_cached_and_invalidated_on_new_grade(self):
+        Grade.objects.create(title='اول', icon='i', color='#111')
+        r1 = self.client.get('/api/grades')
+        self.assertEqual(len(r1.data['grades']), 1)
+
+        with CaptureQueriesContext(connection) as q:
+            self.client.get('/api/grades')
+        self.assertEqual(len([e for e in q if 'content_grade' in e['sql'].lower()]), 0)
+
+        Grade.objects.create(title='دوم', icon='i', color='#222')  # post_save → bump
+        r3 = self.client.get('/api/grades')
+        self.assertEqual(len(r3.data['grades']), 2)
+
+
+class ReadThrottleTests(TestCase):
+    def setUp(self):
+        _django_cache.clear()
+        self.client = APIClient()
+        self.user = User.objects.create(device_id='dev-read-throttle', is_guest=True, name='')
+        self.token = issue_access_token(self.user.id)
+        self.grade = Grade.objects.create(title='g', icon='i', color='#fff')
+
+    def test_read_endpoint_throttles_after_limit(self):
+        # rate پیش‌فرض '120/min' — بعد از ۱۲۰ درخواستِ موفق، ۱۲۱اُم باید 429 بشه.
+        codes = set()
+        for _ in range(122):
+            codes.add(self.client.get(
+                f'/api/home?grade={self.grade.id}',
+                HTTP_AUTHORIZATION=f'Bearer {self.token}').status_code)
+        self.assertIn(429, codes)